In the manufacture of paper, a papermaking furnish is formed into a paper sheet. The papermaking furnish is an aqueous slurry of cellulosic fiber having a fiber content of about 4 weight percent (percent dry weight of solids in the furnish) or less, and generally around 1.5% or less, and often below 1.0% ahead of the paper machine, while the finished sheet typically has less than 6 weight percent water. Hence the dewatering and retention aspects of papermaking are extremely important to the efficiency and cost of the manufacture.
Gravity dewatering is a preferred method of drainage because of its relatively low cost. Other methods can also used for dewatering, for instance vacuum dewatering, pressing, felt blanket blotting and pressing, evaporation and the like. In actual practice a combination of such methods is employed to dewater, or dry, the sheet to the desired water content. An improvement in the efficiency of drainage processes may decrease the amount of water required to be removed by other methods and hence improve the overall efficiency of dewatering and reduce the cost thereof.
Another aspect of papermaking that is extremely important to the efficiency and cost is retention of furnish components on and within the fiber mat. The papermaking furnish represents a system containing significant amounts of small particles stabilized by colloidal forces. The papermaking furnish generally contains, in addition to cellulosic fibers, particles ranging in size from about 5 to about 1000 nm consisting of, for example, cellulosic fines, mineral fillers (employed to increase opacity, brightness and other paper characteristics) and other small particles that generally, without the inclusion of one or more retention aids, would in significant portion pass through the spaces (pores) between the mat formed by the cellulosic fibers on the paper machine.
Greater retention of fines, fillers, and other components of the furnish permits, for a given grade of paper, a reduction in the cellulosic fiber content of such paper. As pulps of lower quality are employed to reduce papermaking costs, the retention aspect of papermaking becomes more important because the fines content of such lower quality pulps is generally greater. Greater retention also decreases the amount of such substances lost to the whitewater and hence reduces the amount of material costs, the impact of increasing levels of such substances with respect to deposition and contamination, the cost of waste disposal and the adverse environmental effects therefrom. It is generally desirable to reduce the amount of material employed in a papermaking process for a given purpose, without diminishing the result sought. Such add-on reductions may realize both a material cost savings and handling and processing benefits.
Another important characteristic of a given papermaking process is the formation of the paper sheet produced. Formation may be determined by the variance in light transmission within a paper sheet, and a high variance is indicative of poor formation. As retention increases to a high level, for instance a retention level of 80 or 90%, the formation parameter generally declines.
Various chemical additives have been utilized in an attempt to increase the rate at which water drains from the formed sheet, and to increase the amount of fines and filler retained on the sheet. For example, high molecular weight polymers can act as flocculants, forming large flocs which deposit on the sheet. They may also aid in the dewatering of the sheet. In conventional programs, the high molecular weight component is added after a high shear point in the stock flow system leading up to the headbox of the paper machine. This is optimal as flocs are formed primarily by a bridging mechanism and their breakdown is a largely irreversible process. For this reason, most of the retention and drainage performance of a flocculant is lost by feeding it before a high shear point. However, feeding high molecular weight polymers after the high shear point often leads to formation problems. The feed requirements of the high molecular weight polymers and copolymers which provide improved retention often lead to a compromise between retention and formation.
There is therefore continuing need to develop new retention aids to increase the efficiency of pulp or paper manufacture.